Rhizoctonia stem canker and
black scurf of potato
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In Michigan, R. solani causes black scurf on tubers (Fig. 1), and stem and stolon canker on underground stems and stolons (Fig. 2), and occurs wherever potatoes are grown. However, R. solani causes economically significant damage only in cool wet soils. In the more southern temperate areas of Michigan, losses from Rhizoctonia are sporadic and only occur when the weather is cold and wet in the weeks following planting. In northern areas, where growers often must plant in cold soils, Rhizoctonia is a more consistent problem. Poor stands, stunted plants, reduced tuber number and size, and misshapen tubers are characteristic of diseases caused by R. solani.
Although black scurf is the most noticeable sign of Rhizoctonia, stem canker (Fig. 2), is the most damaging of the disease as it occurs underground and often goes unnoticed. Early in the season, the fungus attacks germinating sprouts underground before they emerge from the soil (Fig. 3). The sprout may be killed outright if lesions form near the growing tip (Fig 3). Damage at this stage results in delayed emergence and is expressed as poor and uneven stands with weakened plants. Reduction in crop vigor results from expenditure of seed energy used to produce secondary or tertiary sprouts to compensate for damage to primary sprouts. Occasionally, heavily infested potato seed tubers are unable to produce stems. Instead, the tubers will produce stolons with several small tubers. This symptom is referred to as "no top" and can be confused with the same symptom caused by physiologically old seed that has been de-sprouted.
Poor stands may also be mistaken for seed tuber decay, caused by Fusarium or soft rot bacteria, unless plants are dug up and examined. Rhizoctonia does not cause seed decay, damaging only sprouts and stolons. Poor stands and stunted plants can also be caused by blackleg, a bacterial disease that originates from seed tubers and progresses up stems, causing a wet, sometimes slimy rot. In contrast, Rhizoctonia lesions are always dry and usually sunken.
Stolons and roots can also be infected by the pathogen. Early in disease development, stolons, roots and stems have reddish-brown to brown lesions. As lesions mature, they become cankers that are rough, brown and can have craters, cracks or both. Damage varies and can be limited to a superficial brown area that has no discernible effect on plant growth to severe lesions that are large and sunken, as well as necrotic. If cankers are severe they may girdle the stem, interfering with the normal movement of water and carbohydrates throughout the plant.
Late season damage to plants is a direct result of cankers on stolons and stems causing problems with starch translocation. Stolon cankers also affect the shape, size and numbers of tubers produced. If stolons and underground stems are severely infected, the flow of starch from the leaves to the developing tubers is interrupted. This results in small, green tubers, called aerial tubers forming on the stem above the soil (Fig. 4). Formation of aerial tubers may indicate that the plant has no tubers of marketable quality below ground. Interruptions in carbohydrate flow may also result in a stunting or rosetting of the plant. A leaf curl, which can be confused with symptoms of the Potato Leaf Roll Virus, has also been reported in severely infected plants.
Mycelia and sclerotia of R. solani are endemic to Michigan soils, living on organic debris, and can cause disease independently of or in conjunction with seedborne inoculum. Soilborne inoculum is potentially as damaging as seedborne inoculum, but it can cause infection only when the plant organs develop in the proximity of the inoculum. Roots and stolons may be attacked at any time during the growing season, although most infections probably occur in the early part of the plant growth cycle. The plant's resistance to stolon infection increases after emergence, eventually limiting expansion of lesions.
Previous research has shown that soil temperature is a critical factor in the initiation of Rhizoctonia disease in potato, with disease severity being positively correlated with the temperature that is most favorable for pathogen growth. The optimal temperature range for the growth of R. solani AG-3 is 41 to 77°F. Thus, plants will be most susceptible to infection when the soil temperatures are within this critical range. Cool temperatures, high soil moisture, fertility and a neutral to acid soil (pH 7 or less) are thought to favor development of Rhizoctonia disease. Damage is most severe at cool temperatures because of reduced rates of emergence and growth of stems and stolons are slow relative to the growth of the fungus. Wet soils warm up more slowly than dry soils which exasperates damage because excessive soil moisture slows plant development and favors fungal growth. However, it has been shown that high soil temperatures, especially during the early stages of plant development tend to minimize the impacts of R. solani, even when inoculum is abundant.
Sclerotia begin to form late in the season, principally after vine death. The mechanisms involved in sclerotial development on daughter tubers are different from those acting in the infection of the mother plant. The mechanisms which trigger sclerotial formation are not well understood, but they may involve products related to plant senescence. However, daughter tubers produced from infected mother plants do not always become infested with sclerotia.
Following practices that do not delay emergence in the spring minimizes damage caused to shoots and stolons and lessens the chance for infection. Planting seed tubers in warm soil (above 46°F) and covering them with as little soil as possible speeds spout and stem development and emergence reduces the risk of stem canker. Plant fields with coarse-textured soils first because they are less likely to become waterlogged and will warm up faster.
Rhizoctonia does not compete exceptionally well with other microbes in the soil. Increasing the rate of crop residue decomposition decreases the growth rate of Rhizoctonia. Residue decomposition also releases carbon dioxide, which reduces the competitive ability of the pathogen. Since the fungus is not an efficient cellulose decomposer, soil populations are greatly reduced by competing microflora and less disease is observed.
Potatoes should be harvested as soon as skin is set so minimal bruising will occur. The percent of tubers covered with sclerotia increases as the interval between vine kill and harvest is lengthened. Vine removal or burning also reduces the amount of fungus overwintering and thus the amount of inoculum available to infect future potato crops. Do not dump infested tubers on future potato fields as they can become sources of inoculum.
Our preliminary research indicates that it is important to maximize growth of the cover crop using a high seed rate (15 lb. acre or more) and irrigation to improve establishment if rainfall is insufficient. A tiny seed such as mustard cannot be drilled too deep. It appears to establish well if broadcast and harrowed or irrigated into sandy soil. The bio-fumigation benefits of mustard residues are maximized if they are incorporated at or just before flowering. We suggest that residues be mowed and incorporated while still green. Mustards are rapid growing species and can become a weed in a subsequent crop, so it is important not to let this cover crop produce seed (see Cover crop choices for Michigan vegetables. MSU Extension Bulletin E2896).
We are just beginning to understand the exact mechanisms involved in bio-fumigation using mustard cover crops. Initial results from Michigan research indicate that oriental mustard can be used as a cover crop to improve potato root and tuber health. The growth of Rhizoctonia was slowed by 90% in soil amended with oriental mustard cover crop tissue compared to bare soil. A field experiment indicated that tubers of the tablestock variety Onaway had no observable signs of Rhizoctonia when grown after a spring cover crop of oriental mustard. Further research is required to learn more about management practices that optimize the bio-fumigation action of mustard cover crops, but initial results are promising and farmers are encouraged to experiment with brassica cover crops such as oriental and white mustard or oilseed radish to improve soil health.
In-furrow applied fungicides